The scenario is unrealistic, but the energy would be easy to calculate. The energy released by the annihilation of 12 oz (matter + antimatter) is just given by mc2, where c is the speed of light. The energy is around 3*1016 Joules. Expressed in units more familiar for this type of event, that's around 5 megatons of high explosive energy. So it would be like setting off a very large hydrogen bomb. The damage would obviously depend on what was nearby.
Mike W.
(published on 02/21/2015)